This invention relates generally to the non-destructive testing of tapered fastner holes, and more specifically to an improved capacitive probe for detecting flaws in such holes.
The use of capacitive probes in inspecting holes for structural flaws, such as cracks, is not new to the art. Earlier probes with only vertical segments were inserted by hand, removed, turned, and reinserted in the hold; a meter, connected to the probe and measuring capacitance and variations therein, would fluctuate if a flaw was detected in the hole. This probe and method is fairly crude and fails to provide data sufficiently accurate for today's standards. For example, earlier probes are generally insensitive to scoring and other defects caused by drill rotation. In cases where such a flaw can be detected, the earlier probe gives no information relative to the depth of the flaw in the hole or to its vertical dimension. Hence, the distinct condition referred to in the art as "barreling" and "bell-mouthing" could show identical readings.
Furthermore, the earlier probes could not detect whether a hole was too large or too small, the indications from these defects being indistinguishable from those of various other defects.
The invention detects flaws in both the vertical and horizontal direction and discloses their approximate shapes and locations. This additional information aids in determining if the hole is satisfactory and also the cause of the flaw. The invention also measures the extent by which a hole is too large or too small by measuring the amount by which the probe protrudes above or below the work piece.